High intensity infrasonic tunable resonant acoustic test cell

ABSTRACT

An apparatus and method are provided for high intensity acoustic test cells that employ the Helmholtz resonator principle, incorporating at least one moderately-sized volume or test cell that is tunable to a given infrasonic to low-sonic frequency and generates sound of high intensity with very pure sinusoidal waveforms in this test cell or volume, by varying the geometry of a port which is connected to the test volume and open to either atmosphere or a second or input volume and by introducing to the test volume or the input volume a driving acoustic signal at the given tuned frequency. The apparatus and method are used for testing or performing experiments on materials, structures, devices, products, biological entities or humans at high acoustic intensities and frequencies in the low-sonic to infrasonic ranges.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to the field of acoustic testchambers or cells. More particularly, the invention is directed to anapparatus and method for providing an acoustic test chamber or cell thatachieves, in air, very high-intensity infrasonic to low-sonicfrequencies in moderately large test volumes with very pure sinusoidalwaveforms.

[0003] 2. Description of Related Art

[0004] High-intensity acoustic test chambers are well known in the art.Previous high-intensity acoustic test chambers capable of operation atfundamental frequencies below 100 Hz include high-intensityflow-modulator-driven non-resonant chambers, standing-wave resonantchambers driven by flow modulators or loudspeakers, and piston-drivensealed chambers.

[0005] Each of these known high-intensity acoustic test chambers hassignificant limitations. High-intensity flow-modulator-drivennon-resonant chambers as devices are not capable of efficient operationor the production of reasonably undistorted sound (sine waves) belowabout 30 Hz.

[0006] Standing-wave resonant chambers driven by flow modulators orloudspeakers, by their nature, have strongly non-uniform acoustic fieldsin their test volumes and require that the test chamber have a longdimension of at least one-half the wavelength of the lowest usablefrequency. That is, their long dimension must be at least 11 m's at 30Hz.

[0007] Piston-driven sealed chambers require a mechanical drive toaccelerate and decelerate a piston which serves as one wall of a testchamber or test cell. This acceleration/deceleration takes place at veryhigh rates which restricts these devices to very low frequencies andsmall test volumes, typically less than 1 m³.

SUMMARY OF THE INVENTION

[0008] The present invention generates continuous high-intensityacoustic fields with clean sinusoidal waveforms, in air, in a moderatelylarge volume, and in the infrasonic to low-sonic frequency range (1 Hzto 30 Hz).

[0009] Embodiments of the present invention employ a test volume as partof a Helmholtz resonator that may include one or more volumes.

[0010] For both single-volume and multi-volume embodiments, generationof an infrasonic to low-sonic (e.g., 1 Hz to 30 Hz), veryhigh-intensity, spectrally pure acoustic field in volumes of useful size(e.g., 5 m³) is accomplished by using the volumes themselves as parts ofa Helmholtz resonator. These volumes are each directly driven at achosen frequency and intensity by an external acoustic energy source.

[0011] In one embodiment of either a single or multiple volume testcell, each volume is directly driven at a chosen frequency and intensityby a modulated air or gas flow introduced into one of the volumes. Inanother embodiment of a single or multiple volume test cell, each volumeis directly coupled to acoustic transducers. In either embodiment theacoustic field in a given test volume can be tuned to a predetermineddriving frequency by varying the geometry of an associated duct/tuningport connected to said volume but otherwise not directly connected tothe acoustic energy source. The intensity and spectral purity of theacoustic signal in each volume are enhanced by the resonance of itsassociated duct/tuning port.

[0012] In another embodiment of a multi-volume test cell, each testvolume is isolated from an acoustic energy source by means of dividingeach Helmholtz resonator volume into two volumes (input volume and testvolume) in which the input and test volumes are connected to one anotherby a duct/tuning port. Air is exhausted from the input volume to theexterior through a long duct or other high acoustic mass. Isolation ofthe test volume eliminates any possibly undesirable contaminants orcharacteristics of the air or gas flow (e.g., noxious gases, excessivelylow or high temperatures) associated with that flow. Further, isolationeliminates the unidirectional gas flow from the acoustic energy sourcethrough the resonator duct/tuning port and consequent acoustic losses inthe duct associated with turbulence and loss of acoustic mass.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 illustrates an embodiment of the present inventionemploying a single volume acoustic test cell according to the presentinvention.

[0014]FIG. 2 illustrates another embodiment of the present inventionemploying a dual-volume Helmholtz resonator test cell driven by acompressed-gas source through a flow modulator or other acoustic source.

[0015]FIG. 3 illustrates another embodiment of the present inventionemploying an electrical analog equivalent circuit of the dual volumeacoustic test cell illustrated in FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] A first embodiment of the present invention, illustrated in FIG.1, comprises a rigid airtight chamber 10 enclosing a test volume 20,typically about 5 m³. A high-intensity repetitive acoustic signal or airpressure variation, typically with a low fundamental or repetitionfrequency (single Hz to tens of Hz) is introduced into the test volume20 of this chamber by means of an acoustic energy source. One such meansis a source of compressed air (air compressor) 30 and a flow modulator40 which periodically varies the flow of compressed air from the sourceinto the volume 20. It is to be understood that the periodic airpressure variation generated by the acoustic energy source means is notrestricted to be sinusoidal. The test volume 20 and an associated tuningport or duct 50 that communicates with outside free air constitute aHelmholtz resonator 60 that can be tuned by means of varying theinternal length and/or cross section of the tuning port 50 to thefrequency that corresponds to the fundamental frequency of theperiodically varying air or gas flow from the flow modulator 40 or otheracoustic source. The intensity and spectral purity of the sound in thetest volume 20 are thereby amplified by the Helmholtz resonance of thevolume 20 tuning port 50 combination. The maximum dimensions of the testvolume 20 are chosen to be less than one-half of the wavelengthcorresponding to the maximum operating acoustic frequency of theapparatus. With this choice of maximum dimensions, standing waves arenot generated in the test volume 20 and the acoustic field in the testvolume 20 is highly uniform.

[0017] Another embodiment of the present invention is illustrated inFIG. 2 and operates in a similar manner. The apparatus illustrated inFIG. 2 includes a rigid chamber 70 enclosing two volumes 80 and 90connected by a duct/tuning port 100 which rigid chamber and tuning portas a unit act as a Helmholtz resonator that is tuned by varying thelength and/or cross-section of the duct/tuning port 100. In thisembodiment the second (test) volume 90, communicates with the first(input) volume 80, only by means of the duct/tuning port 100 and (withthe possible exception of an optional relatively low-volumepositive-pressure ventilating air input with very high acoustic massand/or resistance 200) is airtight to air and sound flow to the outside.The unidirectional, or DC, component of the air flow from the flowmodulator 40 or other acoustic source and air compressor 30 is exhaustedto the outside free air through a long pipe of small diameter 110 that,by virtue of its high acoustic mass, passes only acoustic energy atfrequencies below the operating frequencies of the system.

[0018] An electrical circuit analog that illustrates the principle ofthe two-volume device, and that may be used to calculate its operatingcharacteristics, is illustrated in FIG. 3. The volumes constituteacoustic compliances that correspond to electrical capacitances 120 and130. The acoustic masses of the duct/tuning port and DC exhaust ventcorrespond to electrical inductances 140 and 150, respectively. LoAcoustic losses correspond to the resistances 160-190, with the flowmodulator or other acoustic source being represented by a periodicallyvarying pressure in series with an acoustic loss corresponding toresistance 160. The operating frequency of the circuit is determined bythe series combination of the volume compliances 120 and 130 with theduct/tuning port mass 140.

[0019] It is to be understood that an electrical circuit analog of thefirst embodiment, as illustrated in FIG. 1, is a simplification of FIG.3 in which elements 130, 140, 180 and 190 are eliminated, the compliance120 becomes that of the single test cell or chamber, and the flow ventmass 150 and vent acoustic loss 170 become the tuning port mass andloss, respectively. In this case, the operating frequency of the circuitis determined by the parallel combination of the chamber compliance 120and the vent/port mass 150.

[0020] From the foregoing it will be obvious to one skilled in the artthat numerous modifications and variations can be made without departingfrom the spirit and scope of the novel aspects of the current invention.For example, the same arrangements of components, appropriately sized,may be applied at frequencies outside the 51 Hz to 30 Hz infrasonic tolow-sonic frequency range and there may be more than one external sourcefor acoustic fields coupled to a multi-cell acoustic test apparatus. Itis to be understood that no limitation of the scope of the presentinvention with respect to the specific embodiments illustrated isintended or should be inferred, but the scope of the present inventionis to be defined solely by the attached claims.

What is claimed is:
 1. A method for providing an acoustic test cell witha periodic high intensity acoustic field, comprising the steps of: (a)supplying a chamber encompassing a volume as an acoustic test cell; (b)employing an external source to provide a periodic high intensityacoustic field, said acoustic field having a fundamental frequencydetermined by its period and an intensity; and (c) directly couplingsaid external source to said volume at a selected frequency andintensity to provide in said volume a periodic high-intensity acousticfield.
 2. A method according to claim 1, wherein said method furthercomprises the steps of: (d) providing a tuning port connected to saidvolume to form a Helmholtz resonator comprising said tuning port andsaid chamber, said tuning port being not directly connected to saidexternal source; and (e) tuning said acoustic field within said volumewith said tuning port.
 3. A method according to claim 2, wherein saidexternal source further comprises: a source providing a source flow ofone of air and gas into each said volume; and a flow modulator forvarying said source flow.
 4. A method according to claim 2, wherein saidexternal source is an acoustic transducer.
 5. A method according toclaim 2, wherein step (e) further comprises varying the geometry of saidtuning port.
 6. A method for an acoustic test cell according to claim 3,further comprising the steps of: (f) dividing said volume into an inputvolume and a test volume; (g) isolating said test volume from saidsource flow; (h) connecting said input volume and said test volume bysaid tuning port; and (i) exhausting air from said input volume to theexterior through a high acoustic mass unit.
 7. A method for an acoustictest cell according to claim 6, wherein said high acoustic mass unit isa long duct.
 8. A method for an acoustic test cell according to claim 2wherein said acoustic field lies in the infrasonic to low-sonicfrequency range of 1 Hz to 30 Hz.
 9. A method for an acoustic test cellaccording to claim 2 wherein said volume is preferably 5 m³.
 10. Anacoustic test cell apparatus employing a periodic high intensityacoustic field, said apparatus comprising: (a) a chamber encompassing avolume; (b) means for generating a periodic high-intensity acousticfield within said volume having a frequency and an intensity; (c) anexternal source directly coupled to said volume for providing saidperiodic high intensity acoustic field; and (d) a tuning port connectedto said volume for tuning said frequency of said high intensity acousticfield within said volume to a predetermined frequency and intensity saidtuning port being not directly connected with said external source. 11.The apparatus of claim 10, wherein: said test chamber is rigid andairtight; said acoustic field is continuous; and said tuner and saidvolume form a Helmholtz resonator.
 12. The apparatus of claim 11,wherein at least one said external source comprises: (a) a sourceproviding a source flow of one of air and gas into said volume; and (b)a modulator for varying said source flow.
 13. The apparatus of claim 11,wherein said external source is an acoustic transducer.
 14. Theapparatus of claim 11, wherein said tuning port comprises a variablegeometry for tuning said acoustic field.
 15. The apparatus of claim 11,wherein said acoustic field lies in the infrasonic to low-sonicfrequency range of 1 Hz to 30 Hz.
 16. The apparatus of claim 11, whereinsaid volume is preferably 5 m³.
 17. The apparatus of claim 11, whereinsaid volume further comprises: (a) an input volume and a test volume,said test volume being acoustically isolated from both said source flowand said input volume and connected to said input volume by saidassociated tuning port; and (b) a high acoustic mass means forexhausting air from said input volume to the exterior.
 18. The apparatusof claim 17, wherein at least one of said high acoustic mass meanscomprises a long duct.
 19. A method for an acoustic test cell accordingto claim 10 wherein said acoustic field lies in the infrasonic tolow-sonic frequency range of 1 Hz to 30 Hz.
 20. A method for an acoustictest cell according to claim 10 wherein said volume is preferably 5 m³.21. An electrical circuit which constitutes an analog of the apparatusof claim 17, comprising: (a) an air flow modulator circuit providing acontinuous field, comprising: (i) an AC power source providing a voltagesource representing a periodically varying gas pressure source, and (ii)a resistance element representing the flow resistance of a gas flowmodulator having said resistance element connected in series with saidAC power source; (b) an input volume circuit in series with said fieldsource, comprising: (i) an inductance element representing a highacoustic mass in series with a resistance element that representsacoustic losses associated with said acoustic mass, (ii) a capacitanceelement representing an input volume in parallel with said high acousticmass, and (iii) a resistance element representing acoustic loss in aninput volume in parallel with said input volume; (c) a tuning portcircuit in series with said input volume circuit and comprising: (i) aninductance element providing a tuning port mass, and (ii) a resistanceelement representing acoustic loss in a tuning port in series with saidinductance element; (d) a test volume circuit in series with said tuningport circuit and comprising: (i) a capacitance element representing atest volume, and (ii) a resistance element representing acoustic loss ina test volume in parallel with said capacitance element; whereincontinuous DC current flow is varied periodically by said flow modulatorcircuit and is directly coupled with said input volume, said inputvolume is vented by said high acoustic mass and is tuned by said tuningport to produce a predetermined AC voltage representing an acousticsignal in said test volume.